German Patent No. 20 19 937 describes a safety barrier circuit structure which utilizes an in-line control element, formed as a bipolar power transistor, having a collector which is directly connected to the input of the safety barrier, and an emitter which is connected to the output through a current measuring resistor. The standards of safety barriers are set forth in e.g. DIN EN 50020.
Leading from the collector to the base of the the in-line transistor is a resistor that at the same time is the operating resistor of a bipolar control transistor, the collector of which is applied to the base of the in-line transistor and the emitter of which is connected directly to the barrier output. Its base is connected to the emitter of the bipolar power transistor, so that the voltage drop at the emitter resistor of the power transistor is the control voltage for the second, control transistor.
In normal operation, the in-line transistor is kept in the conductively saturated state by the operating resistor connected between its base and its collector, because the control transistor is in the blocking state as long as the voltage drop at the emitter resistor of the in-line transistor remains below the threshold voltage of the control transistor - in a silicon transistor this is approximately 0.6 V. As soon as the current through the safety barrier reaches a value that causes the voltage drop at the emitter resistor of the in-line transistor to increase to above the threshold voltage of the control transistor, the control transistor begins to conduct and assumes a more or less large current component, which flows from the operating resistor and which, when the control transistor was blocked, flowed solely into the base of the in-line transistor. With an increasing current transfer by the control transistor, the in-line transistor representing the actual power element becomes gradually blocked, so that the current through the safety barrier can now increase only very slowly, if there is an increasing load on the output voltage of the safety barrier. In that operating condition the safety barrier has a very high internal resistance, whereas previously there was only a low internal resistance, which was substantially due to the emitter resistor for the power transistor and to the specifications of the turned-on power transistor.
The current increase that is also possible upon attaining the limit condition when there is an increasing load at the barrier output limits the useful operating range of the known safety barrier, because when the output voltage at the barrier output is decreasing, increasing maximum currents arise. To remain on the safe side in that situation, that is, so as to have low output voltages that do not exceed the maximum permissible current, the limiting point for large output voltages must be at correspondingly low currents, which means that with full output voltage only a low output current is possible.
Furthermore, the circuit of the known safety barrier, even in the conductively saturated state, has a relatively high internal resistance, resulting from the size of the emitter resistor plus the output internal resistance of the in-line transistor. The size of the emitter resistor is a function of the current limitation and the threshold voltage of the control transistor, depending on the required operating point. The lower the threshold current of the barrier becomes, the larger the emitter resistor will accordingly be.
Under some circumstances, both resistors may falsify the measurement result considerably, e.g., if a measuring converter is connected to the barrier output, this converter imposing a current between 4 and 20 mA when a minimum supply voltage is applied to the connected current circuit as a function of the measurement variable (for example, flow or pressure).
It is important here to keep the in-line resistor of the barrier incorporated in the transmitter current circuit small enough that even at the maximum current (20 mA) a sufficiently large supply voltage for the transmitter will remain. The situation becomes still more unfavorable if two circuits such as those described above are connected in series to improve error redundancy, as in the known safety barrier, because in that case the sizes of the emitter resistors as well as of the internal resistances are consequently summed together.
The above-described relatively flat limiting characteristic of the known safety barrier is due, first, to the finite amplification of finite steepness of the control transistor, which does not operate digitally but instead is controlled to be conductive and therefore assures a controlled turn-off state of the in-line transistor only to the extent that the voltage drop at the emitter resistor of the in-line transistor is also increasing. A further cause for the relatively flat characteristic after the limiting condition is the increasing current through the control transistor in the limiting situation.